全文获取类型
收费全文 | 38613篇 |
免费 | 3032篇 |
国内免费 | 2571篇 |
专业分类
化学 | 26294篇 |
晶体学 | 453篇 |
力学 | 1596篇 |
综合类 | 235篇 |
数学 | 4228篇 |
物理学 | 11410篇 |
出版年
2024年 | 52篇 |
2023年 | 330篇 |
2022年 | 775篇 |
2021年 | 988篇 |
2020年 | 884篇 |
2019年 | 962篇 |
2018年 | 863篇 |
2017年 | 830篇 |
2016年 | 1351篇 |
2015年 | 1306篇 |
2014年 | 1747篇 |
2013年 | 2536篇 |
2012年 | 2917篇 |
2011年 | 3245篇 |
2010年 | 2267篇 |
2009年 | 2148篇 |
2008年 | 2771篇 |
2007年 | 2482篇 |
2006年 | 2481篇 |
2005年 | 2118篇 |
2004年 | 1809篇 |
2003年 | 1485篇 |
2002年 | 1552篇 |
2001年 | 1050篇 |
2000年 | 907篇 |
1999年 | 630篇 |
1998年 | 420篇 |
1997年 | 343篇 |
1996年 | 404篇 |
1995年 | 274篇 |
1994年 | 276篇 |
1993年 | 265篇 |
1992年 | 210篇 |
1991年 | 173篇 |
1990年 | 162篇 |
1989年 | 145篇 |
1988年 | 105篇 |
1987年 | 96篇 |
1986年 | 70篇 |
1985年 | 92篇 |
1984年 | 57篇 |
1983年 | 53篇 |
1982年 | 69篇 |
1981年 | 66篇 |
1979年 | 52篇 |
1978年 | 60篇 |
1976年 | 48篇 |
1975年 | 40篇 |
1974年 | 43篇 |
1973年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
灰色模型的最优化及其参数的直接求法 总被引:2,自引:0,他引:2
基于灰色模型的内涵表达式和白化方程响应式均为等比级数的观点,提出了一种不用求ago值、均值,不涉及灰色微分方程,白化微分方程概念,直接求灰色模型参数a,c的方法,通过此方法建立的新模型不仅从理论上可保证是在满足给定评价标准为模拟绝对误差平方和最小(或模拟相对误差平方和最小)、给定精度条件下的最优化模型,从而结束了灰色模型只有更优,没有最优的历史.并从理论上证明了新模型具有白化指数律重合性、白化系数律重合性,伸缩变换一致性.最后通过实例编程验证该方法具有可操作性,且预测精度高,效果好. 相似文献
122.
黎勇 《数学物理学报(B辑英文版)》2007,27(2):437-448
In this article, two relaxation time limits, namely, the momentum relaxation time limit and the energy relaxation time limit are considered. By the compactness argument, it is obtained that the smooth solutions of the multidimensional nonisentropic Euler-Poisson problem converge to the solutions of an energy transport model or a drift diffusion model, respectively, with respect to different time scales. 相似文献
123.
The solvability of a class of forward-backward stochastic differential equations (SDEs for short) over an arbitrarily prescribed time duration is studied. The authors design a stochastic relaxed control problem, with both drift and difftusion all being controlled, so that the solvability problem is converted to a problem of finding the nodal set of the viscosity solution to a certain Hamilton-Jacobi-Bellman equation. This method overcomes the fatal difficulty encountered in the traditional contraction mapping approach to the existence theorem of such SDEs. 相似文献
124.
Wei‐Lie Xiao Jian‐Xin Pu Rui‐Rui Wang Liu‐Meng Yang Xiao‐Li Li Sheng‐Hong Li Rong‐Tao Li Sheng‐Xiong Huang Yong‐Tang Zheng Han‐Dong Sun 《Helvetica chimica acta》2007,90(8):1505-1513
Seven new highly oxygenated nortriterpenoids, rubriflorins D–J ( 1 – 7 ), were isolated from the leaves and stems of Schisandra rubriflora, and their structures were elucidated on the basis of extensive analysis of spectroscopic data. These new compounds feature the opening of ring A compared with related known nortriterpenoids isolated from the genus Schisandra and showed weak activity against HIV‐1. 相似文献
125.
126.
127.
Various phase behavior of blends of poly(vinyl ether)s with homologous acrylic polymers (polymethacrylates or polyacrylates) were examined using differential scanning calorimetry, optical microscopy (OM), and Fourier‐transformed infrared spectroscopy. Effects of varying the pendant groups of either of constituent polymers on the phase behavior of the blends were analyzed. A series of interestingly different phase behavior in the blends has been revealed in that as the pendant group in the acrylic polymer series gets longer, polymethacrylate/poly(vinyl methyl ether) (PVME) blends exhibit immiscibility, upper critical solution temperature (UCST), and miscibility, respectively. This study found that the true phase behavior of poly(propyl methacrylate)/PVME [and poly(isopropyl methacrylate)/PVME)] blend systems, though immiscible at ambient, actually displayed a rare UCST upon heating to higher temperatures. Similarly, as the methyl pendant group in PVE is lengthened to ethyl (i.e., PVME replaced by PVEE), phase behavior of its blends with series of polymethacrylates or polyacrylates changes correspondingly. Analyses and quantitative comparisons on four series of blends of PVE/acrylic polymer were performed to thoroughly understand the effects of pendant groups in either polyethers (PVE's) or acrylic polymers on the phase behavior of the blends of these two constituents. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1521–1534, 2007 相似文献
128.
Conducting drop tests to investigate impact behavior and identify failure mechanisms of small-size electronic products is
generally expensive and time-consuming. Nevertheless, strict drop/impact performance criteria for hand-held electronic products
such as cellular phones play a decisive role in the design because they must withstand unexpected shocks. The design of product
durability on impact has heavily relied on the designer's intuition and experience. In this study, a reliable drop/impact
simulation for a cellular phone is carried out using the explicit code LS-DYNA. Subsequently globallocal experimental verification
is accomplished by means of high-speed photography and impact response measurement. Using this methodology, we predict potential
damage locations in a cellular phone and compare them with real statistical data. It is envisaged that development of a reliable
methodology of drop/impact simulation will provide us with a powerful and efficient vehicle for improvement of the design
quality and reduction of the product development cycle. 相似文献
129.
Xiu‐Li Wang Ke‐Ke Yang Yu‐Zhong Wang Zhi‐Xuan Zhou Yong‐Dong Jin 《Journal of polymer science. Part A, Polymer chemistry》2004,42(14):3417-3422
A new biodegradable starch graft copolymer, starch‐g‐poly(1,4‐dioxan‐2‐one), was synthesized through the ring‐opening graft polymerization of 1,4‐dioxan‐2‐one onto a starch backbone. The grafting reactions were conducted with various 1,4‐dioxan‐2‐one/starch feed ratios to obtain starch‐g‐poly(1,4‐dioxan‐2‐one) copolymers with various poly(1,4‐dioxan‐2‐one) graft structures. The microstructure of starch‐g‐poly(1,4‐dioxan‐2‐one) was characterized in detail with one‐ and two‐dimensional NMR spectroscopy. The effect of the feed composition on the resulting microstructure of starch‐g‐poly(1,4‐dioxan‐2‐one) was investigated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3417–3422, 2004 相似文献
130.
In this paper we present numerical methods for computations of nonequilibrium hypersonic flow of air around bodies including chemical reaction effects and present numerical result of the flow over concave corners. We developed implicit finite difference method to overcome numerical difficulties associated with the lack of resolution behind the shock and near the body. Using our method we were able to find details of the flow properties near the shock and body and were able to continue the computation of the flow for a long distance from the corner of the body. 相似文献